Polyimides derived from condensation of pyromellitic dianhydride with various diamines have been shown to be electrochemically active. Films of these materials coated on the surface of solid electrodes, immersed in electrolyte solutions of dipolar aprotic solvents undergo two distinct reversible redox processes, involving addition of one or two electrons per repeat unit. These processes can be carried out completely and stoichiometrically for films as thick as 20 μm. Spectrophotometric studies and comparison with model compounds lead to the following conclusions: (i) ion permeability, which is a necessary component of the electrochemical charging and discharging processes, is related to solvent swelling and is, in turn, strongly influenced by the physical history of the polymer, (ii) redox activity involves localized molecular electronic states of the pyromellitic diimide group, (iii) there is no evidence for electronic interactions between pyromellitimide groups along a given polymer chain, and (iv) spectroscopic evidence indicates that weak interactions within the bulk of the polymer give rise to detectable inhomogeneities in optical or electrochemical properties of individual functional groups as a function of the charge on groups in their environment.